Generally a DC voltage or a DC current is needed for charging a battery. A charging station for a battery of an electric vehicle is as a rule connected to a 3-phase AC mains connection, so that an AC/DC conversion initially has to be performed. In such cases the conventional choice is not to perform the conversion in the electric vehicle but in the charging station, in order to avoid the electric vehicle itself having to be equipped with an alternating current-direct current converter (AC-DC converter).
A corresponding DC charging station for electric vehicles generally has at least one 3-phase AC mains connection, an input filter and an isolating transformer, an AC-DC converter and also a battery connection.
The AC-DC converter, especially when the charging station is to be used both for charging and for discharging a battery, is a controlled converter. 6-pulse or 12-pulse thyristor bridges or what are known as IGBT bridges (“insulated-gate bipolar transistor” or bipolar transistor with insulated gate electrode), which are typically operated with a clock rate in the kHz range can for example be used as forms in which such a converter can be realized.
In order to reduce so-called “ripple” on DC current and DC voltage at the output of the converter, especially in the case of thyristor bridges, an output filter in the form of a passive electrical network is frequently connected between converter and battery.
FIG. 1 shows a known circuit diagram of a DC charging station with a 3-phase mains connection 1, an input filter 2, an isolating transformer 3, an AC-DC converter 4, an output filter 5, and also a battery connection 6, via which the battery to be charged 7 can be connected to the charging station. The fuses on the input and output side of the converter 4 not identified by any reference number in FIG. 1 are typically present for the protection of the converter 4.
If the battery 7 were to be permanently connected to the output filter 5, i.e. hard wired, and not able to be connected or disconnected by a plug, the output-side circuitry of the AC/DC converter 4 would be unproblematic. Such a case is for example presented in U.S. 2010/0019737 A1. But since the battery 7 in the case of an electric vehicle is sensibly connected via an electric coupling 6, for example a plug connection, to the output terminals of the output filter 5 of the DC charging station, problems arise if, when the coupling or connection is made, the battery voltage U2 and the output voltage U1 of the charging station, i.e. the output voltage of the output filter 5, differ. In this case, when the connection between battery 7 and charging station is made via the components of the output filter 5, a battery charging or discharging current can flow, the maximum amount of which can amount to a multiple of the maximum permissible charging or discharging current and which can therefore lead to damage to all electric components through which current flows, even to destruction of the battery 7.
The problem described can in principle typically be avoided or at least ameliorated with the aid of the following measures:
(i) Dispensing with the output filter: this variant is principally conceivable with sufficiently rapidly-clocked and regulated IGBT converters, however at the expense of a greater current ripple on the battery charging or discharging current compared to the embodiment with output filter. This higher ripple can reduce the battery life. Furthermore faster clocking of the converter leads to higher losses in the converters. The aim is therefore, as regards energy efficiency, to make the clock frequencies as small as possible and the regulation times as large as possible. It is not conceivable to dispense with the output filter in the case of thyristor bridge converters. The resulting ripple of the battery charging or discharging current would be unacceptably large.
(ii) Expansion of the LC output filter by a resistor, which is connected in series with the capacitor C. This represents a current-limiting measure for the battery charging or discharging current, which is produced as a result of an initial voltage difference between U1 and U2. This measure however has the effect of making the effectiveness of the output filter worse. Furthermore such a resistor represents an additional loss source during the charging or discharging process, associated with a deterioration in the efficiency of the charging station.
(iii) Insertion of a diode 5 between output filter 5 and battery terminals 6. According to the related art (see for example the Chademo standard), this diode is inserted into the plus line and is referred to as the reverse current diode. Such a diode prevents a change in direction of the current flow and limits the integral of current multiplied by time to the maximum charge of the capacitor in the output filter 5. The disadvantages of a reverse current diode are:
(a) The reverse current diode is an additional source of losses, wherein the loss power is equal to the charge current times the voltage drop of typically 0.9 V to 1.5 V over the reverse current diode.
(b) A charging station with reverse current diode is not capable of feeding back, i.e. with such a charging station the battery can only be charged, but not discharged.